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Re: Cisco ACL bug when using VPN crypto engine accelerator, PPPoE dialer or ip route-cache
From: Ilker Temir <itemir () cisco com>
Date: Thu, 15 May 2003 18:56:36 +0200 (CEST)

Hash: SHA1

This is in response to the e-mail sent by Olivier. The original e-mail is
available at

Hi Olivier,

We can confirm that the inbound ACL is processed twice for IPSec traffic
which is the root cause of the issue you have described.

This issue is being addressed by the Cisco Bug ID CSCdz54626. Cisco's
Product Security Incident Response Team (PSIRT) was not previously aware
of it. We are now addressing it with due priority.

While this issue does cause administrative overhead to configuration,
there is no significant security impact.

Permitting internal networks in the inbound ACL may be exploited to inject
spoofed packets into the network. However this has no practical impact
while using static crypto maps. With static crypto maps, the unencrypted
traffic will be dropped even if it passes the inbound ACL.

In the case of dynamic crypto maps, in order to bypass the inbound ACL to
inject spoofed packets, an attacker would also require control of the
neighboring routers that are connected to the interface where the inbound
ACL is applied, or the medium in between the neighbors.

Protecting our customers' networks is very important for us and we are
always open for vulnerability reports regarding any Cisco product.
Such reports should be directly sent to psirt () cisco com or
security-alert () cisco com 

Thank you again for your report,


- --
Ilker Temir
Incident Manager, PSIRT
Cisco Systems, Inc.
+32 2 704-6031

On Wed, 14 May 2003, Olivier wrote:

Platform Cisco 1760 dual Ethernet


Environment: Site to site VPN for small offices.

ACL are not properly parsed as soon as you enable:

crypto engine accelerator
PPPoE dialer
Ip route-cache

Without the feature mentioned above, you can apply an ACL on the outside
interface allowing only inbound ISAKMP and IPSEC traffic.


ip access-list extended Block-Inbound-unwanted-Trafic

 permit udp host eq isakmp

 permit esp 100.100. 100.0 host

 deny   ip any any log

If you activate the crypto engine, the ACL is parsed as well on decrypted
traffic which forces you to allow as well all traffic for the decrypted
I.E. If you are using 10.x addressees internally and the subnet for your Soho LAN. Can be worst if you have a huge network
inside where you would prefer to add permit ip  any

ip access-list extended Block-Inbound-unwanted-Trafic
 permit udp host eq isakmp
 permit esp 100.100. 100.0 host
 permit ip  <----------- () %#$%@
 deny   ip any any log

This looks pretty bad for a VPN box running a Firewall feature set IOS
seen as the best candidate for VPN for small offices.

The worst is the reply from Cisco:
We will be addressing this in the next few months however
the release time frame could be as late as the end
of the year.

We do have plans to address it but do
not expect it in a released image until the
last calendar quarter of the year. If its possible we
can get it done and released sooner than what I've
mentioned, we will do it, no guarantees however.

We would have hope that they put more resources and concern in solving
security issue.

Version: GnuPG v1.2.1 (SunOS)


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